In recently developed space-saving sound apparatus, a space designated for mounting a speaker is limited. Thus, the speaker is requested to have a compact and flat profile, as well as be light in weight. At the same time, in order to comply with an increased dynamic range of digital sound sources, the speaker is requested to be compatible with large outputs.
A conventional speaker used in the above environment is described referring to FIG. 38 and FIG. 39. FIG. 38 is a cross sectional view of a conventional outer magnet type speaker, while FIG. 39 shows a cross sectional view of a conventional inner magnet type speaker.
Referring to FIG. 38, a conventional outer magnet type speaker comprises a magnetic circuit 51 formed of a bottom plate 42 having a center pole 41; a cylindrical magnet 43 disposed on the bottom plate 42 and an upper plate 44 disposed on the magnet; a frame 52 attached on the magnetic circuit 51; a voice coil 53 having a coil 53a; a damper 54; a diaphragm 55; a dust cap 57; and a lead wire 58 which is connected at one end with the coil 53a, and at another end with a terminal 59, for feeding input signals delivered from outside of the speaker. A conventional inner magnet type speaker of FIG. 39 is structured likewise, except for an existence of a center pole 41, and a positioning of the magnet 43.
The total height of a speaker equals a sum of the total height of magnetic circuit 51, a distance from the upper end of the magnetic circuit 51 to damper 54 including an amplitude margin and a height of diaphragm 55 disposed above the damper 54. This means that if a speaker is to be made more flat, the height above the magnetic circuit 51 needs to be reduced. Namely, the height of diaphragm 55 needs to be reduced. Furthermore, a vertical distance between an outer circumference of damper 54 and the coil 53a is made to be greater than a vertical distance between an outer circumference of an edge 56, or the fixed end of the speaker supporting system, and an outer circumference of the damper 54.
However, a diaphragm 55 of lower height is structurally strong in strength in the vibrating direction. This means that such a diaphragm is not suitable for a high output speaker, and the threshold frequency at high frequency range sound reproduction becomes low. In order to maintain a certain strength with a diaphragm 55 of low profile, the diaphragm 55 needs to have a greater thickness; which leads to an increased weight, and the increased strength inevitably decreases the efficiency of a speaker. Meanwhile, when the vertical distance between the outer circumference of damper 54 and the coil 53a is made to be greater than the vertical distance between the outer circumference of an edge 56 and the outer circumference of the damper 54, the center of gravity of the vibration system shifts towards the coil side, or goes down, which makes the vibrating motion unstable. Therefore, the above-described configuration is not suitable for a high output application; especially, for reproduction of heavy bass sounds where a diaphragm moves in large amplitudes. Furthermore, many of the conventional flat-profile speakers have been exposed during operation to a risk of breakage at the connecting part between the outer circumference of voice coil 53 and the inner circumference of damper 54 due to insufficient adhesive strength.
As is described above, if a speaker is modified into a flat-profile keeping the basic conventional structure as it is, the modified speaker becomes unsuitable for high-output and great-amplitude application.
Another example of a flat-profile speaker is disclosed in Japanese Patent No. 2756037 (International Publication No.: WO90/05435). The speaker is provided in at least one of the magnetic poles with openings, which extend parallel to a magnetic gap and a moving direction of a voice coil. Ribs for fixing the voice coil and a flat diaphragm together are inserted in the opening in order to transfer a vibration of the voice coil to the diaphragm. The diaphragm is substantially thick and has a flat structure, because it is directly connected with the voice coil.
The speaker of Patent No. 2756037, however, can not employ an ordinary voice coil because of the rib. In the speaker, a coil is adhered onto the inside of a bobbin fixed to the diaphragm; which means that it needs an extra step of adhering during assembly of a speaker. Further, because there is no mechanical engagement between the coil and the bobbin, the connecting strength is solely dependent on adhesive strength of the adhesives. As a result, a temperature withstanding capability of a speaker is limited by a heat resistive property of the adhesives. Thus, the speaker of the above-described configuration is not suitable for use in a high input power. Furthermore, the thick diaphragm of the speaker makes it inappropriate for the high frequency sound reproduction.